This application is based on and claims the priority under 35 U.S.C. xc2xa7 119 of German Patent Application 199 50 247.1, filed on Oct. 18, 1999, the entire disclosure of which is incorporated herein by reference.
The invention relates to a method and an apparatus for regulating or controlling the position, attitude and motion of a satellite by detecting reference targets, such as stars for example.
It is generally known in the prior art to use stars as reference targets. These reference targets are detected, and then the detected data relating to targets are evaluated, in order to provide a regulation or control of a satellite. For example, U.S. Pat. No. 4,801,202 (Wilcox) and U.S. Pat. No. 5,821,526 (Krishna) each respectively disclose a method of evaluating star data in such a manner.
A problem or shortcoming of position determinations in space in general, and particularly in connection with the above mentioned prior art references, is that only a relative position of a star or the like with respect to the position of the satellite can be determined. In this context, the respective positions of the satellites and of the stars are only known with a limited degree of accuracy. Moreover, various interfering influences have an effect on a given satellite and thus change the motion of the satellite relative to the desired path, whereby such interfering influences make it more difficult to determine the position of stars relative to the desired satellite position.
In view of the above it is an object of the invention to provide a method and an apparatus for regulating or controlling a satellite, which enables achieving the highest possible accuracy of the position regulation for the satellite. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in an apparatus for controlling or regulating a satellite, comprising: at least one sensor for detecting reference targets such as stars, connected with an arrangement for selecting reference targets, and an arrangement for determining the respective position of the reference targets, which generally is achieved relative to the satellite coordinate system; an arrangement for estimating or evaluating model parameters of the satellite and external interferences that have an influence on the satellite, through parameter identification; a first memory storing a reference target catalog; a second memory storing a reference trajectory, which may be in the form of a concrete description or definition of the reference trajectory path, or of a calculation rule for calculating the reference trajectory; a regulating unit that is connected to actuators or position control units for regulating or controlling the position of the satellite; and a filter which contains models of the satellite motion and of the external interferences that have an influence on the satellite, and which is connected to the arrangement for position determination of the reference targets, the arrangement for estimating or evaluating the model parameters, the first memory, and the regulating unit. As a result of the models, condition or state values for the external interferences and for the satellite motion are provided.
In such an apparatus according to the invention, not only are reference targets, such as stars for example, detected, selected and their positions determined, but also model parameters, such as the moment of inertia of the satellite for example, and parameters characterizing the external interferences that hate an influence on the satellite, such as magnetic forces, gravitational forces, the solar wind, etc., for example, are additionally estimated or evaluated in an appropriate arrangement and can therefore be used for improving the state or condition values which are provided as a result of the models. A reference target catalog provides further references for a position determination and position or attitude regulation. Using the filter, a current or present position of the satellite can be calculated from the estimated values and then compared by using the reference target catalog. By comparison with the reference trajectory stored in the second memory, a position and attitude regulation of the satellite can be achieved using a regulating unit.
In order to simplify the apparatus, it can be provided that the filter, the arrangement for position determination of the reference targets, and the arrangement for selecting reference targets are combined together in an extended filter arrangement. Furthermore, also the arrangement for estimating or evaluating the condition parameters and the external interferences can be integrated in this extended filter arrangement. In this context, either the filter itself can be embodied as a Kalman filter, or the extended filter arrangement can be embodied as an extended Kalman filter.
The actuators or the position control units or the like that are connected to the regulating unit may be any type of engines, for example electrical engines, of the satellite. In order to achieve the finest possible control of the effectiveness of the several engines, the invention further provides that the engines are tiltable or pivotable between a common main thrust direction and a thrust direction that is tilted or inclined relative to the main thrust direction. With such an arrangement, in order to achieve the greatest thrust possible, the respective thrust direction of all the engines is directed in a common direction, or in order to achieve a reduced thrust and finer control thereof, the engines may be tilted or pivoted relative to each other to achieve an effective resultant thrust that is smaller than the sum of the individual thrusts of the individual engines, because components of these individual thrusts partially compensate or counteract each other. Also, with an appropriate adjustment of the engines, instead of a straight linear force being applied to the satellite, it is alternatively possible to exert a rotational moment onto the satellite. Thus, with a suitable engine adjustment, a controlled rotation of the satellite is respectively possible about each of the three axes.
The above objects have further been achieved according to the invention in a method of regulating or controlling a satellite, comprising the following steps: detecting, selecting and determining the position of reference targets such as stars; evaluating or estimating model parameters of the satellite and external interferences that have an influence on the satellite; calculating current or present position, attitude and motion conditions of the satellite and external interference conditions on the basis of the estimated model parameters by means of dynamic models of the satellite motion; comparing the detected positions of selected reference targets with stored reference target data; and correcting the calculated position, attitude and motion conditions dependent on the results of the comparison.
Thus, on the one hand, the present or current model parameters are taken into account in the context of an estimation, from which present position, attitude and motion conditions can be calculated with the aid of dynamic models, and external interference conditions can be calculated also with the aid of corresponding models. In this context, these position, attitude and motion conditions and also the external interference conditions, to the extent applicable in a given case, can be further corrected to approach the actually existing conditions by carrying out a comparison of the measured reference targets with the stored reference target data. Since, however, the stored reference target data, such as star charts for example, generally themselves are subject to a certain degree of inaccuracy or uncertainty, it is advantageous that a correction and therewith an improvement of the stored reference target data can be carried out on the basis of the comparison and therewith the newly measured reference target data. Thus, the above described process not only more exactly or accurately determines the present position, attitude and motion conditions of the satellite and also the external interference conditions, in any case where applicable, while taking into consideration the stored reference target data, but also in the opposite sense it is possible to carry out a more exact determination of the stored reference target data.
The results of these correction steps can thereby be still further improved in that the individual calculation and correction steps are multiply or repetitively carried out in an iterative manner. Thereby, also the estimated model parameters are newly determined dependent on the corrected results and adapted to the corrected values. Then, on the basis of this corrected estimation, a renewed calculation of the present position, attitude, and motion conditions of the satellite, and also the external interference conditions, if applicable, is carried out, and then again a renewed comparison of the measured reference target data with the stored reference target data is carried out. Then, once again, dependent on the comparison results, a correction step for the position, attitude and motion conditions and also for the external interference conditions as applicable, as well as for the stored reference target data, can be carried out.
The position regulation or control of the satellite especially shall serve to maintain the satellite as well as possible on the ideal motion path, i.e. trajectory or orbit, that has been prescribed for it. This position regulation is achieved dependent on a comparison of the calculated present position, attitude, and motion condition of the satellite with respect to a reference trajectory. Then, a position regulation or control of the satellite can be achieved using a regulating unit and actuators, based on and corresponding to any deviation of the present position, attitude and motion conditions of the satellite away from the reference trajectory.
Basically, it is also possible according to the invention that an improvement of the reference target data is not only achieved by a calculation and correction on-board the satellite, but also the pertinent improved and corrected reference target data can be transmitted to the satellite from an external source, for example a ground station. The stored reference target data can be used to select a certain number or set of particular reference targets after a measurement of reference targets has been carried out. In this context, the determined position data or patterns of the detected reference targets can be compared to stored reference target data or patterns, and then a selection among the reference targets is carried out. For example, those reference targets of which the deviation from the stored reference target data is smaller than a predefined threshold value can be selected in the selection process. As an example in this context, only those reference targets, such as stars for example, of which the determined position only slightly deviates from the stored reference target data, such as star charts for example, will be taken into consideration. If the reference targets are determined in such a manner, it is possible to reduce the data for the selected targets, because once an unambiguous identification and allocation of the reference targets to already-known targets has been achieved, then only certain characterizing data, such as star position, brightness or magnitude and possibly also the color spectrum must be further processed.